1. Field of the Invention
The present invention is directed to a method for establishing a route via a communication network comprising a plurality of network nodes connected to one another.
2. Description of the Related Art
In communication networks, a distinction is made between connectionless and connection-oriented communication networks dependent on the topology of the communication networks. In connectionless communication networks, a signalling phase preceding a message transmission between comm terminal devices allocated to the communication network is omitted, a communication connection between the communication terminal devices being set up in the framework of said signalling phase. In the framework of the signalling phase, a layer-3 connection (switching layer) is established according to the OSI reference model (Ōpen {overscore (S)}ystems Īnterconnection), i.e. the communication connection between the communication terminal devices is established by section-by-section linking of individual layer-2 connections (security layer) according to the OSI reference model—also frequently referred to as point-to-point connection in the literature.
Connectionless communication networks are, for example, local networks—often abbreviated as LAN ({overscore (L)}ocal Ārea {overscore (N)}etwork) in the literature—with a ring or bus architecture. In a connectionless communication network, a message package to be transmitted is transmitted to each communication terminal device allocated to the communication network. The decision as to which communication terminal device further-processes or, discards the received message packet is made by the recipient the he message packet.
Similarly, communication services wherein a message transmission ensues without setup of a point-to-point communication connection established in the framework of a signalling phase are referred to as connectionless services. Such connectionless services, however, can only be realized within local networks (LANs) with corresponding network topology.
In the framework of a message communication between two communication terminal devices allocated to different comm networks, the lack of signalling must be compensated by corresponding measures given a network interworking. This compensation ensues with what are referred to as routers, with which a linking of communication networks with different network topology is realized.
With respect to its critical function, a router realizes the layer 3 (switching layer) of the OSI reference model, whereby networks with different topology of the layers 1 (bit transmission layer) and 2 (security layer) are physically connected with the assistance of a router. In order to switch message packets between the networks connected to a router, the address particulars contained in routing information of the message packets—particularly a destination and a source address—are interpreted and evaluated by a control means located in the router. As such the router evaluates the layer-3 addresses contained in a message packet to be transmitted and converts these into corresponding layer-2 addresses. The layer-2 connections (i.e. the connections to be respectively established between two network nodes arranged in a network) required for a layer-3 connection (i.e. for the terminal device connection between the communication terminal devices) are established on the basis of these layer-2 addresses. Subsequently, the message packets are converted protocol-suited for a transmission and arc transmitted via the layer-2 connections that have been established.
In the framework of a message transmission via an ATM-based communication network (Āsynchronous {overscore (T)}ransfer {overscore (M)}ode), a signalling phase preceding the message transmission between is required for setting up a communication connection between two communication terminal devices, i.e. an ATM-based communication network is a connection-oriented communication network.
Before the beginning of the message transmission, connection tables with switching information composed of a virtual channel identification and of a virtual path identification are established in the pertinent ATM network node in an ATM-based communication network. In the connection tables, a VCI value is allocated to the virtual channel identification and a VPI value is allocated to the virtual path identification. The switching information entered in the connection tables define how the virtual paths—or, virtual transmission channels contained in the virtual paths—of the incoming and outgoing connections at an ATM network node are allocated to one another by the signalling, i.e. which input is connected to which output in switching-oriented terms.
For example, European Published Application EP 0 406 842 A2 discloses a communication network based on the asynchronous transfer mode, whereby switching information in the framework of a signalling phase are entered in the traversed network nodes of the communication network with a connection setup message communicated from a source to a destination communication terminal device and a confirmation message subsequently communicated from the destination to the source communication terminal device.
In the cell-based data transmission method known as asynchronous transfer mode (ATM), data packets of a fixed length, what are referred to as ATM cells, are used for the data transport. An ATM cell is composed of a five byte long cell header—what is referred to as the header—containing switching data relevant for the transport of an ATM cell and of a 48 byte long payload cell, what is referred to as the payload. ATM cells transmitted via the virtual connections essentially comprise switching data composed of a VPI value and a VCI value in the cell header. The data deposited in the header are processed at the input of an ATM network node, i.e. the switching data arranged therein are acquired and interpreted. Subsequently, the ATM cells are switched to an output representing a specific destination on the basis of the switching information stored in the connection table, being switched thereto by a switching network module arranged in the ATM network node.
In order to be able to emulate connectionless services in an ATM-based communication network, a conversion of the switching data deposited in the cell header of a communicated ATM cell must ensue in every ATM network node arranged in the ATM-based communication network, so that the required switching capacity or, calculating performance increases greatly in an ATM network node. Given an increased data volume, this can lead to a communication delay—often referred to as “delay” in the literature—, so that, for example, connectionless services cannot be utilized within the scope of applications having real-time demands.
In order to reduce the required calculating outlay in an ATM network node, “ATM networks—concepts, protocols, applications”, Addison-Wesley, 3rd edition, 1998, ISBN 0-201-17817-6 discloses that connectionless services be realized via specific, what are referred to as “CL servers” ({overscore (C)}onnection {overscore (L)}ess {overscore (S)}erver). These CL servers are connected to one another and undertake the address conversions needed for an emulation of connectionless services in an ATM-base communication network analogous to the methods that are implemented in routers. Given the methods employed here, however, a processing or, communication delay can also arise given an elevated data volume.